Gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine assemblies to these high temperatures. As a result, turbine blades must be made of materials capable of withstanding such high temperatures. In addition, turbine blades often contain cooling arrangements for additional thermal protection.
Typically, turbine blades are formed from a root at one end for engaging a shaft and an elongated radial portion forming an airfoil that extends outwardly from a platform coupled to the root. The blade is ordinarily composed of a tip opposite the root, a leading edge, and a trailing edge. The interior structure of most turbine blades typically contains cooling channels forming part of a cooling arrangement. The cooling channels in the blades receive air from the compressor of the turbine engine and pass the air through the blade. The cooling channels often include multiple flow paths. Centrifugal forces and air flow at boundary layers may result in localized hot spots. Localized hot spots, depending on their location, can reduce the useful life of a turbine blade.
The cooling scheme for a turbine blade will depend upon its location within the turbine. The temperature of the working fluid will decrease as the fluid expands through the turbine and imparts its energy to the machine in the form of shaft power. Thus, the first row of blades is subjected to the highest gas temperature, and each successive row is subjected to a sequentially lower gas temperature. In addition, each successive row of blades gets longer in the radial direction, and may include more taper in cross-sectional area from root to tip, and may include more twist about its radial axis from root to tip. Row 1 blades of current generation industrial gas turbines are coated with a ceramic thermal barrier coating material and also include internal cooling fluid passages; whereas no ceramic coating material and no active cooling is needed for Row 4 blades of the same machines. U.S. Pat. No. 6,910,864 discloses a cooling scheme for a Row 2 industrial gas turbine blade consisting of a series of generally radially oriented cooling holes passing through the blade interior.